Tissue is well known in the art and a staple of everyday life. Tissue is commonly divided into two uses--toilet tissue and facial tissue. Both require several attributes in order to be accepted by the consumer. One of the most important attributes is softness.
Softness is a subjective evaluation of the tactile sensation the user feels when handling or using the tissue. Softness cannot be directly measured. However relative softness values can be measured in panel score units (PSU) according to he technique set forth in commonly assigned U.S. Pat. No. 5,354,425 issued Oct. 11, 1994 to Mackey et al., except that the samples are not allowed to be judged equally soft. This patent is incorporated herein by reference. Softness has been found to be related to 1) the surface topography of the tissue, 2) the flexibility of the tissue, and 3) the slip-stick coefficient of friction of the surface of the tissue.
Several attempts have been made in the art to improve softness by increasing the flexibility of the tissue. For example, commonly assigned U.S. Pat. No. 4,191,609 issued to Trokhan has proven to be a commercially successful way to increase flexibility through a bilaterally staggered arrangement of low density regions. However, it has been well recognized in the art that multi-density tissues, which provide very high and commercially successful flexibility and softness, have an inherently distinctive topography.
However, improving, and even maintaining, softness by providing a smoother surface topography has proven to be elusive. The reason for this elusiveness is the trade-off between the smoother surface topography and increased density. Typically, densification increases fiber to fiber contacts, potentially causing bonding at the contact points. This negatively impacts flexibility and hence softness. This interdependent density/softness relationship has been referred to as virtually axiomatic in the commonly assigned U.S. Pat. No. 4,300,981 issued Nov. 17, 1994 to Carstens. The Carstens '981 patent also discusses the PSU softness measurement and is incorporated herein by reference. This relationship is also stated in competitive European Patent Application 0 613 979 A1, published Sep. 7, 1994, as increased void volume (i.e., decreased density) correlates with improved softness. Unfortunately, this trade-off has inimical effects for tissue products sought by the consumers.
Unexpectedly, applicants have found a way to decouple the prior art relationship between density and softness. Accordingly, it is now possible to improve the surface topography of tissue without encountering the concomitant loss of softness that occurs in the prior art. Therefore, softness levels, previously unattainable at relatively high densities, are possible with the present invention. Also, unexpectedly, absorbency is maintained at the higher density. This is contrary to prior art beliefs, as illustrated by European Patent Application 0 616 074 A1, which holds lower density results in more bulky and absorbent sheets.
Further unexpectedly, it has been found necessary to utilize a multidensity substrate to make tissue according to the present invention. This is unexpected because multidensity tissue, particularly through air dried tissue, generally has a lesser density than conventionally dried tissue having a uniform density throughout. Thus, rather than using high density tissue as a starting point in the calendering process, one must utilize relatively lower density tissues as the starting point.